Light emitting device

1. A light emitting device comprising: a light emitting structure comprising a plurality of compound semiconductor layers; a light extraction structure on the light emitting structure; and an electrode on the light extraction structure, wherein the compound semiconductor layers comprise: an active layer; a first conductive semiconductor layer under the active layer; and a second conductive semiconductor layer on the active layer, wherein the light extraction structure comprises a plurality of first layers and a plurality of second layers which are alternately disposed with each other to have a negative refraction index, and wherein the electrode passes through the light extraction structure and contacts the light emitting structure.

2. The light emitting device of claim 1 , wherein the light extraction structure is disposed on one of the first and second conductive semiconductor layers.

3. The light emitting device of claim 1 , wherein the first layer is a dielectric layer and the second layer is a metal layer.

4. The light emitting device of claim 3 , wherein the dielectric layer is formed of one of a transparent oxide-based material, a transparent nitride-based material and a transparent carbide-based material.

5. The light emitting device of claim 1 , wherein the first layer is a lowermost layer of the light extraction structure and the second layer is an uppermost layer of the light extraction structure.

6. The light emitting device of claim 1 , wherein the first and second layers each has a thickness of 0.2λ p , wherein λ p denotes a plasma frequency of the second layer.

7. The light emitting device of claim 1 , wherein an upper layer of the first and second layers covers a side surface and a top surface of a lower layer and a portion extending from the upper layer makes contact with a top surface of one of the first and second layers.

8. The light emitting device of claim 7 , wherein thicknesses of the first and second layers are equal to each other in a vertical direction.

9. The light emitting device of claim 8 , wherein widths of the first and second layers are equal to each other in a horizontal direction.

10. The light emitting device of claim 9 , wherein the widths of the first and second layers are greater than the thicknesses of the first and second layers.

11. The light emitting device of claim 7 , wherein thicknesses of the first and second layers are different from each other in a vertical direction.

12. The light emitting device of claim 11 , wherein widths of the first and second layers are different from each other in a horizontal direction.

13. The light emitting device of claim 12 , wherein the widths of the first and second layers are greater than the thicknesses of the first and second layers.

14. The light emitting device of claim 7 , wherein, when a portion of the upper layer is the first layer, the portion of the upper layer forms an ohmic contact with one of the first and second conductive semiconductor layers.

15. The light emitting device of claim 7 , wherein, when a portion of the upper layer is the second layer, the portion of the upper layer forms a Schottky contact with one of the first and second conductive semiconductor layers.

16. The light emitting device of claim 1 , wherein a light incident into the light extraction structure from the light emitting structure is reflected toward an outside with respect to a vertical interface of the light extraction structure.

17. The light emitting device of claim 1 , wherein the light emitting structure generates an ultraviolet light having a wavelength in a range of 365 nm to 488 nm.

18. A light emitting device comprising: a substrate; a first conductive semiconductor layer on the substrate; an active layer on the first conductive semiconductor layer; a second conductive semiconductor layer on the active layer; and a light extraction structure on the second conductive semiconductor layer, wherein the first conductive semiconductor layer is an n-type semiconductor layer and the second conductive semiconductor layer is a p-type semiconductor layer, wherein the light extraction structure comprises a plurality of first layers and a plurality of second layers which are alternately disposed with each other to have a negative refraction index, and wherein the first layer is a dielectric layer and the second layer is a metal layer, and at least one of the first and second layers has a thickness of 0.2λ p wherein λ p denotes a plasma frequency of the second layer.

19. A light emitting device comprising: a first conductive semiconductor layer; an active layer under the first conductive semiconductor layer; a second conductive semiconductor layer below the active layer; an electrode layer under the second conductive semiconductor layer; and a light extraction structure on the first conductive semiconductor layer, wherein the first conductive semiconductor layer is an n-type semiconductor layer and the second conductive semiconductor layer is a p-type semiconductor layer, wherein the light extraction structure comprises a plurality of first layers and a plurality of second layers which are alternately disposed with each other to have a negative refraction index, and wherein the first layer is a dielectric layer and the second layer is a metal layer, and at least one of the first and second layers has a thickness of 0.2λ p , wherein λ p denotes a plasma frequency of the second layer.